T1-weighted inversion recovery (IR) imaging is a common method used in Magnetic Resonance Imaging (MRI) that is frequently performed in conjunction with suppression of fat signal. T1 is the time constant that describes the rate of recovery of longitudinal magnetization. T1-weighted IR is used for imaging different pathophysiologies in different regions of the body, including but not limited to the heart, brain, and vasculature. This method can be used with or without the administration of a T1-shortening contrast agent. Fat suppression methods are used to make fat appear dark in MR images so that other bright structures can be visualized without being confounded by bright fat, for example infarcted myocardium in contrast enhanced delayed enhancement imaging of the heart. Known different fat suppression methods, when combined with T1-weighted IR (for example in contrast-enhanced MR angiography or delayed enhancement) work poorly or not at all.
A known inversion recovery pulse is used in MRI to create T1-image contrast so image elements with a short T1 value (“short T1-species”) appear bright, and long-T1 species appear dark in such images. Fat appears bright due to its short T1 value and its abundance in most patients. This can be a problem in image interpretation as it may be difficult to discriminate fat from the other short T1-species present in the image and this is important for making a clinical diagnosis. Another such short T1-species for example is blood with contrast agent in contrast-enhanced MR angiography, or infarcted (dead) myocardium (heart tissue) in contrast-enhanced delayed enhancement. Using delayed enhancement as a specific example, a bright fat signal can obscure the presence of bright infarcted myocardium, or fat can be mistaken for infarct. In arrhythmogenic right ventricular dysplasia (ARVD), the discrimination between fat and equally bright scar tissue is important and is difficult with known systems.
A common application of fat signal manipulation is fat suppression. One known fat suppression method uses a fat-frequency selective saturation recovery (SR) pulse played immediately before readout of data. This method is not optimal in clinical segmented inversion recovery protocols which acquire only a fraction of the data known as a segment (typically 21-29 lines per segment). In the common linear reordering scheme, by the time the k-space center is acquired, the fat magnetization has significantly recovered due to its short T1 (T1 of fat=230 ms at 1.5 T, 290 ms at 3 T) and is thus poorly suppressed. The fat signal has recovered even more in the case of single-shot imaging where the center of k-space is typically acquired 100 ms to 120 ms after the fat suppression pulse. This method works poorly at 1.5 T and better, but still not proficiently, at 3 T field strength. Centric reordering improves this fat suppression method, but is prone to image artifacts.
A STIR (short tau inversion recovery) pulse sequence provides another known fat suppression method. This method is used in connection with turbo-spin echo (TSE) readout and dark-blood (DB) preparation. One non-frequency selective but usually spatially-selective IR (NFSIR) pulse is played timed to null the fat at the beginning of the TSE readout and not the center of k-space. STIR suppresses fat well due to the nature of the TSE readout; the first TSE readout pulse is a 90 degrees pulse that “locks in” the nulled fat signal (after that pulse, the longitudinal relaxation of fat is irrelevant for the remainder of the readout). Gradient echo (GRE, Siemens proprietary name Flash, fast low angle shot) and steady state free precession (SSFP, Siemens proprietary name TrueFisp, true fast imaging with steady precession) readouts do not have this “lock-in” property and thus require different timing between the NFSIR pulse and the beginning of the readout. Such timing restricts the maximum number of lines per segment, often below a clinically useful value. Thus, the STIR sequence works only in combination with the TSE readout.
Furthermore, STIR works with a single inversion time which is used to null fat. It is substantially impossible to apply an additional non-frequency selective IR pulse to impart T1-contrast, as the application of both pulses unfavorably alters the image contrast and prevents the suppression of fat signal. In addition, a dark blood (DB) preparation is required to be used with the STIR method to avoid image artifacts. DB preparation is restricted to non-contrast agent applications due to timing limitations. Therefore, STIR may only be used without contrast agent.
A SPAIR (Spectral Selection Attenuated Inversion Recovery) or SPIR (Spectral Presaturation Inversion Recovery) sequence provides other known fat suppression methods. These methods work in the same ways as STIR with a difference being that a NFSIR pulse is replaced by a SPAIR or a SPIR pulse. Both pulses are fat-frequency selective and spatially non-selective. The problems are similar to those of STIR, but both pulses can be used as a fat-frequency selective inversion pulse.
Other known methods that render fat dark in the image are Dixon-type methods, variable projection (VARPRO), and other estimation methods. These methods suppress fat well, but require time-consuming post-processing. A known method that uses different echo times (TE) in conjunction with an SSFP readout does not work together with GRE readout. A system according to invention principles addresses the deficiencies of known systems and the combination of IR and fat saturation.